This invention relates generally to polymer displays which have light emitting layers that are semiconductive polymers.
Polymer displays use layers of light emitting polymers. Unlike liquid crystal devices, the polymer displays actually emit light which may make them advantageous for many applications.
Generally polymer displays use at least one semiconductive conjugated polymer sandwiched between a pair of contact layers. The contact layers produce an electric field which injects charge carriers into the polymer layer. When the charge carriers combine in the polymer layer, the charge carriers decay and emit radiation in the visible range.
One semiconductive conjugated polymer that may be used in polymer displays is poly(p-phenylenevinylene) (PPV) which emits green light. Another polymer which emits red-orange light is poly(methylethylhexyloxy-p-phenylenevinylene) (MEH-PPV).
Other polymers of this class are also capable of emitting blue light. In addition nitrile substituted conjugated polymers may be used in forming polymer displays.
It is believed that polymer compounds containing vinyl groups tend to degrade over time and use due to oxidation of the vinyl groups, particularly in the presence of free electrons. Since driving the display with a current provides the free electrons in abundance, the lifetime of the display is a function of total output light. Newer compounds based on fluorine have similar degradation mechanisms that may be related to chemical purity, although the exact mechanism is not yet well known in the industry. In general, polymer displays have a lifetime limit related to the total output light. This lifetime is a function of intrinsic lifetime and the display usage model.
Overdriving the polymer display can increase its useful lifetime because as the display degrades, its output light is increased. overdriving may be done by increasing the display's brightness. However, degradation may introduce output non-uniformity errors. If some of the pixels of the display are degraded non-uniformly, simply overdriving the display does not solve the non-uniform degradation problem. Even after overdriving, some pixels will be brighter than other pixels.
Thus, there is a continuing need for ways of controlling polymer displays that account for non-uniform degradation of individual pixels.